FIG. 14A is a view showing a structural example of a voltage controlled ring oscillator. The voltage controlled ring oscillator is a Voltage Controlled Oscillator (VCO). A plurality of differential amplifiers 1401 are ring-connected. A plurality of variable resistors 1402 are respectively connected to the plurality of differential amplifiers 1401. A plurality of current sources 1403 are respectively connected to the plurality of differential amplifiers 1401. A CR ring element 1404 has each one of the differential amplifier 1401, the variable resistor 1402, and the current source 1403. Four of the CR ring elements 1404 are ring-connected, for instance.
FIG. 14B is a circuit diagram showing a structural example of the CR ring element 1404 in FIG. 14A. The CR ring element 1404 has a bias unit 1431 and an oscillating unit 1432. Hereinafter, an MOS field-effect transistor is simply referred to as a transistor. The bias unit 1431 has a P-channel transistor 1411 and an N-channel transistor 1412.
The oscillating unit 1432 inputs differential signals from a non-inverting input terminal I+ and an inverting input terminal I− to amplify them, and outputs the amplified differential signals from a non-inverting output terminal O+ and an inverting output terminal O−. The differential signals are two signals whose phases are mutually inverted by 180°. The N-channel transistor 1422 corresponds to the current source 1403 in FIG. 14A, and constitutes a current mirror circuit with the transistor 1412. When a current Il flows through the transistor 1412, a current Is flows through the transistor 1422. To the non-inverting output terminal O+ and the inverting output terminal O−, a parasitic capacitance 1425b and a parasitic capacitance 1425a are respectively connected. The non-inverting input terminal I+ is connected to a gate of an N-channel transistor 1421a, and the inverting input terminal I− is connected to a gate of an N-channel transistor 1421b. P-channel transistors 1424a and 1423a are connected to the transistor 1421a, and constitute a load resistor. P-channel transistors 1424b and 1423b are connected to the transistor 1421b, and constitute a load resistor. The transistors 1423a and 1423b are variable resistors controlled by a voltage Vcntl. In the CR ring element 1404, a delay amount of the output signal is determined by a CR, that is, a capacitance and a resistor. By varying the variable resistors 1423a and 1423b with the voltage Vcntl, an oscillation frequency of the ring oscillator can be controlled.
As described above, the voltage controlled ring oscillator is composed of the oscillating unit 1432 which applies positive feedback by connecting a plurality of stages of Current Mode Logic (CML)-type ring elements (CR delay elements), and the bias unit 1431 which supplies a bias voltage to the ring elements. The control voltage Vcntl is input into the bias unit 1431, and the bias unit 1431 outputs the bias voltage which is in proportion to the voltage Vcntl. The bias voltage controls a tail current source 1422 and the transistors 1423a and 1423b being the load resistors in a CML circuit. There is a method for controlling only either the load resistors 1423a and 1423b or the tail current source 1422, but, in order to make an amplitude constant without depending on the oscillation frequency, a method for controlling both of them is commonly adopted.
FIG. 15 is a graph showing a relation between the control voltage Vcntl and the oscillation frequency fosc. Since the control voltage Vcntl is controlled after passing through one stage of the transistor 1411 in the bias unit 1431, it is affected by a threshold voltage Vth of the transistor. The transistor 1411 is connected at a source thereof to a power supply voltage Vdd, and connected at a drain thereof to a gate of the transistor 1422. In the ring oscillator, there is a problem that the threshold voltage Vth becomes a hindrance for obtaining a tuning range, so that the tuning range has to be secured with the voltage in which the threshold voltage Vth is subtracted from the power supply voltage Vdd. The tuning range of the control voltage Vcntl required to realize a target oscillation frequency 1501 which meets the specifications, is a tuning range 1502. Depending on a process variation and the like, a characteristic varies between a characteristic FAST and a characteristic SLOW. A characteristic TYP is a typical characteristic when the threshold voltage Vth takes a typical value. The characteristic FAST is a characteristic when the threshold voltage Vth varies at low frequencies. The characteristic SLOW is a characteristic when the threshold voltage Vth varies at high frequencies. The ring oscillator is affected largely by the process variation.
When trying to secure the target tuning range 1502 by including the process variation and variations of the power supply voltage Vdd, a temperature, and the like, the minimum of the characteristic is determined at the characteristic SLOW with the narrowest band, which consequently brings about a problem that a gain of the VCO becomes very high at the characteristic TYP. The gain of the VCO corresponds to an inclination of the characteristic. When the gain of the VCO is increased, the VCO itself becomes highly sensitive, so that it is forced to respond sensitively to the power supply voltage variation and the variation of the control voltage Vcntl. This leads to generate a jitter, which deteriorates the performance of a PLL (Phase Locked Loop) circuit when the VCO is applied thereto.
Further, along with the advance in technology, a voltage scaling (lowering the voltage) is in progress. In the ring oscillator, the voltage scaling is a big problem. While the power supply voltage is scaled, the threshold voltage Vth of the transistor is not scaled, so that the value Vdd-Vth being the tuning range 1502 becomes smaller. When a circuit is composed using a voltage of 1V or lower in the future, the tuning range 1502 cannot be secured by an existing circuit system, which can be easily predicted.
As described above, since the tuning range 1502 is determined by a value Vdd-Vth, the existing ring oscillator has dealt with it by increasing the gain of the VCO. However, to increase the gain of the VCO means to simultaneously increase a noise (jitter), and it also suggests that the design becomes further difficult to cope with the power supply voltage scaling in the future.
Further, an electrostatic capacitance type weight sensor is known wherein the sensor has a variable capacitor formed of a fixed electrode and a movable electrode in which a capacitor having a predetermined electrostatic capacitance is inserted between a fixing metal frame and the fixed electrode, to thereby largely reduce a variation of oscillation frequency offset values and the sensitivity variation caused by a stray capacitance depending on an environmental change and a change with time.
Further, a self-excited oscillation circuit is known wherein the oscillation circuit has a CR oscillating unit in which a variable capacitance diode is used.
Furthermore, a ring oscillator is known in which a plurality of differential gain stages are connected in a ring shape via resistor elements, and a band-pass filter is respectively connected between each of inputs of the differential gain stages and a reference potential.